This invention relates to shielded co-axial and other electrical cables and to methods of making these cables by which interference immunity may be increased. As is well known, a co-axial cable with two or more braided wire screens as the outer conductor over the dielectric has better performance in this respect than a cable having only one braided wire screen. Moreover, it has been shown in the Paper No B11-3 entitled "On the interference immunity of co-axial cables" and read at the EUROCON 71 Conference IEEE Region 8, that the interposition of a layer of magnetic material between these braids makes a very marked improvement in the interference immunity by improved radio frequency shielding. The magnitude of the improvement in interference immunity is a function of the permeability and thickness of the magnetic material used and of the thickness of the effective air gap in the magnetic path presented by the magnetic material.
It is of course important that the addition of magnetic material does not seriously impair the flexibility of the cable inherent in its construction and hence it has been the practice to employ magnetic material in the form of a layer of tape of high permeability such as "mu-metal." It has been found that the way the tape layer is applied can have a large influence on the mechanical and electrical properties of the finished cable.
For example, if the magnetic tape is wound on with the edges of the tape butting between the turns (so as to minimise the air gap) any bending of the cable tends to induce a buckle in the tape with a consequential degradation of both its mechanical and magnetic properties. This drawback may be avoided if the tape is wound on with a gap between successive turns and although this method introduces a larger air gap than with a butted winding, a second layer of tape may be applied in staggered relation to the first. A further alternative, and one with which the invention is directly concerned, is to wind on the tape with a significant overlap, so minimising the air gap. Under these conditions the trailing edge of the tape must stretch an amount proportional to the tape thickness to allow a continuous winding operation without any buildup of diameter. Difficulties arise in applying a tape in this way for if an annealed tape is wound on with a high tension so that it is stretched beyond its elastic limit both edges of the tape will stretch unless very careful control of tension is exercised. The trailing edge of the tape being applied to the cable must wind on to a cable whose radius is increased by the thickness of the immediately preceding lap of tape, hence the trailing edge will stretch a little more than the leading edge. Unfortunately this stretching degrades the magnetic properties and the resulting tight binding effect results in a somewhat stiff cable.